Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches_count

From: Dave Chinner <david@fromorbit.com>
To: Nag Avadhanam <nag@cisco.com>
Cc: Daniel Walker <danielwa@cisco.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Khalid Mughal <khalidm@cisco.com>,
	xe-kernel@external.cisco.com, dave.hansen@intel.com,
	hannes@cmpxchg.org, riel@redhat.com,
	Jonathan Corbet <corbet@lwn.net>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] kernel: fs: drop_caches: add dds drop_caches_count
Date: Tue, 16 Feb 2016 19:22:41 +1100	[thread overview]
Message-ID: <20160216082241.GY19486@dastard> (raw)
In-Reply-To: <alpine.LRH.2.00.1602152135280.29586@mcp-bld-lnx-277.cisco.com>

On Mon, Feb 15, 2016 at 09:57:42PM -0800, Nag Avadhanam wrote:
> On Mon, 15 Feb 2016, Dave Chinner wrote:
> >So, to pick a random active server here:
> >
> >		before		after
> >Active(file):   12103200 kB	24060 kB
> >Inactive(file):  5976676 kB	 1380 kB
> >Mapped:            31308 kB	31308 kB
> >
> >How much was not reclaimed? Roughly the same number of pages as the
> >Mapped count, and that's exactly what we'd expect to see from the
> >above page walk counting code. Hence a slightly better approximation
> >of the pages that dropping caches will reclaim is:
> >
> >reclaimable pages = active + inactive - dirty - writeback - mapped
> 
> Thanks Dave. I considered that, but see this.
> 
> Mapped page count below is much higher than the (active(file) +
> inactive (file)).

Yes. it's all unreclaimable from drop caches, though.

> Mapped seems to include all page cache pages mapped into the process
> memory, including the shared memory pages, file pages and few other
> type
> mappings.
> 
> I suppose the above can be rewritten as (mapped is still high):
> 
> reclaimable pages = active + inactive + shmem - dirty - writeback - mapped
> 
> What about kernel pages mapped into user address space? Does "Mapped"
> include those pages as well? How do we exclude them? What about
> device mappings? Are these excluded in the "Mapped" pages
> calculation?

/me shrugs.

I have no idea - I really don't care about what pages are accounted
as mapped. I assumed that the patch proposed addressed your
requirements and so I suggested an alternative that provided almost
exactly the same information but erred on the side of
underestimation and hence solves your problem of drop_caches not
freeing as much memory as you expected....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com